1. Field of Invention
The present invention relates to steel for nitriding and a sliding member made of the steel with the nitriding. More particularly, the present invention relates to steel where nitriding or soft-nitriding is conducted on the surface thereof. The steel with the nitriding or soft-nitriding exhibits high wear-resistance and fatigue-strength and is appropriate for the sliding member.
2. Description of Related Art
There are a number of parts which are required to satisfy sliding property and fatigue resistance property simultaneously, such as a spring, a piston ring and a gear. The scuff resistance and the wear-resistance properties are collectively referred as the sliding property. Generally speaking, the sliding property and the fatigue resistance property are contradictory to each other as follows. An increase in hardness results in improvement of the sliding property but incurs embrittlement and strength reduction of the material. Since fatigue strength is usually recognized to be a half of the tensile strength, the strength reduction readily result in reduction of the fatigue strength. The nitriding treatment is used at present to solve the contradiction as described above. That is, a product made of steel for nitriding, is subjected to nitriding on the sliding surface thereof Surface hardness of the steel with the nitriding is greatly enhanced as compared with that of the inside of the steel. As a result, the sliding property such as wear resistance and scuff resistance properties is greatly improved.
In addition to the hardness increase, large residual compressive stress generates on the surface of the steel with the nitriding. The fatigue strength is, therefore, greatly improved as compared with that of the steel without the nitriding. When the steel surface with the nitriding is further subjected to shot-peening or carburization, large further compressive stress is superimposed so that the parts having higher fatigue strength are provided.
As the steel for nitriding, it is known heretofore to use a martensitic 13Cr stainless steel as well as low-alloyed steel with the addition of Al and Cr.
Heretofore, almost no discussions or consideration has been made as to nitriding structure to enhance fatigue strength to a required level. In other words, if the fatigue strength by the nitriding is unsatisfied, the steel with nitriding is ordinarily subjected to post-nitriding treatment such as shot-peening or carburization. The post-nitriding treatment increases, however, processing steps and cost.
It is, therefore, an object of the present invention to provide such a steel for nitriding that the required level of the fatigue strength can be attained by nitriding without post-nitriding treatment such as shot-peening and carburization.
It is also an object of the present invention to provide a sliding-member having satisfactory fatigue strength without post-nitriding treatment such as shot-peening and carburization.
In accordance with the objects of the present invention, there is provided a steel for nitriding, which consists of from 0.5 to 1.0% of C, 1.0% or less of Si, from 0.3 to 1.0% of Mn, from 5.0 to 12.0% of Cr, from 0.5 to 2.0% of Mo, from 0.1 to 0.3% of V, the balance being Fe and unavoidable impurities.
There is also provided a sliding member made of the steel with nitriding or soft-nitriding mentioned above.
According to an embodiment of the sliding member of the present invention, the nitriding layer comprises crystalized grains, (iron) compound layers precipitating along the boundaries of the crystal grains, and precipitates consisting essentially of carbonitrides dispersed within the crystal grains and having less than 10 xcexcm in size, and, further, the area percentage of the precipitates from 1 to 10 xcexcm in size is 5% or less.
The fracture toughness of the nitriding layer of the steel according to the present invention is high. The sliding member with the nitriding has thus high fatigue resistance even if without post-nitriding treatment. The present invention is hereinafter described with reference to the composition.
A part of the alloyed Cr substitutes for Fe of the iron lattices, and Fe and Cr form a substitutional solid solution. The solute Cr of the substitutional solid solution promotes the nitriding. The other part of Cr reacts with C and forms chromium carbide in the steel. Fine carbo-nitrides are formed in the nitriding layer after the nitriding or soft-nitriding. As a result, the matrix in the nitriding layer is moderately hardened by the fine carbo-nitrides. The matrix in the nitriding layer provides resistance against propagation of cracks generated inside the material, as described more in detail hereinbelow. This resistance against crack propagation and the fatigue strength attained by the present invention are higher than that of the steel member having less than 5% of Cr, or that of the steel member without nitriding. When the Cr content is 12.0% or more, since almost all of the Cr carbides is converted to carbo-nitrides after nitriding, coarse carbo-nitrides or a coalescent structure of fine carbo-nitrides is easily formed. As a result, the fatigue strength is lowered. The Cr content is, therefore, 12% or less. A preferable Cr content is from 7 to 11%. In the surface vicinity of the steel (supposed nitriding layer), where the nitriding layer is to be formed, the following structure is preferable. That is, the size of the Cr carbide in the surface layer (supposed nitriding layer) is 10 xcexcm or less, and the area ratio of the Cr carbide from 1 to 10 xcexcm in size is 5% or less. The steel for nitriding having such fine carbide-structure can be produced for example by means of increasing the cooling speed in casting.
A part of C is dissolved in the matrix of the steel for nitriding and raises the hardness by the interstitial solution hardening, while the other part of C reacts with Cr and other carbide-forming elements and forms carbides. The wear resistance is thus enhanced. The C content must therefore be 0.5% or more. On the other hand when the C content is 1.0% or more, carbides prominently tend to so coarsen as to impede the nitriding. A more significant fact is that the cold workability is extremely impaired at a C content of 1.0% or more. The C content is not less than 0.5% and not more than 1.0%. A preferable C content is from 0.7 to 0.8%.
Si is added as a deoxidizing agent and is dissolved in the Fe matrix, too. This Si solute improves the resistance against thermal setting. Si may, therefore, be contained in some degree. However, when the Si content is more than 1.0%, the cold workability is impaired due to embrittlement. The Si content is therefore 1.0% or less.
Mn is also added as a deoxidizing agent as is Si. Mn content of 0.3% or more is necessary for the deoxidation. When the Mn content is 1.0% or more, oxidation resistance as well as the hot workability and cold workability are impaired. The Mn content is, therefore, not less than 0.3% and not more than 1.0%.
Mo in an amount of 0.5% or more is necessary for suppressing the temper softening during the nitriding. Mo forms the carbides in small size and enhances the hardness. Mo is, thus, effective for enhancing the wear resistance. However, when Mo, which is a strong carbide-former, is added in an amount of 2.0% or more, the coarse carbides are formed. As a result, a structure having high fatigue resistance cannot be obtained. The Mo content is, therefore, not less than 0.5% and not more than 2.0%.
A trace amount of V greatly enhances nitriding velocity and hardness of the nitriding layer. This effect is not realized when the V content is less than 0.1%. On the other hand, when the V content is 0.3% or more, vanadium carbides are formed in the grain boundaries, thereby lessening the toughness. The V content is, therefore not less than 0.1% and not more than 0.3%.
A sliding member according to the present invention comprises a nitriding layer having from 5 to 200 xcexcm of thickness, on at least the outer peripheral sliding surface of the steel. The precipitates mainly consists of carbo-nitrides and is dispersed in the crystal grains of the matrix of the nitriding layer. The matrix phases is martensite, in which solute Cr is contained, and the like. Others are carbides and the like. In the present invention, the precipitates are controlled to 10 xcexcm or less in size, so as to enhance the sliding property of the nitriding layer itself. In addition, the area ratio of the precipitates not less than 1 xcexcm and not more than 10 xcexcm in size is controlled to less than 5%, so as to suppress mutual coalescence of the carbo-nitrides.
Relatively large iron compounds precipitates along the grain boundaries. When the Cr carbides exist in the microstructure is coverted to Cr carbonitrides during the nitriding. A portion of the carbon of the carbides becomes excessive. Such excessive carbon is expelled from the carbides toward the grain boundaries and reacts with Fe and N at the grain boundaries. The resultant compound is a very hard compound. The grain-boundary compound is three-dimensionally continuous because of the reasons described above. For a crack originated at the non-metallic compound to propagate through the nitriding layer, it must cross through the grain-boundary compound. In other words, this compound is effective for impeding the propagation of cracks, since this compound precipitates along the grain boundaries of the nitriding layer. Specifically, the uniformly precipitated compound indicates a network structure. As a result, the fatigue resistance is furthermore enhanced.
The nitriding methods, which can be applied to the steel according to the present invention, are varied, such as gas-nitriding, soft-nitriding and salt-bath nitriding.